1. Field of the Invention
The present invention relates to the art of stencil printing, and more particularly, relates to a rotary stencil printer.
2. Description of the Prior Art
As a type of stencil printer there is a rotary stencil printer comprising a printing drum supported to be rotatable about a central axis thereof and having a cylindrical outer surface adapted to bear a perforated stencil sheet as attached therearound and formed with openings for supplying ink therethrough toward an inside surface of said perforated stencil sheet, and a back press roller supported to be rotatable about a central axis thereof extending in parallel with said central axis of said printing drum and having a cylindrical outer surface which defines a nip area with said cylindrical outer surface of said printing drum therebetween for nipping and transferring a printing sheet therethrough, characterized in that said printing drum has an outer peripheral wall portion substantially made of only a net material constructed by weaving or knitting filaments to provide said cylindrical outer surface thereof formed with said openings, and an internal press roller is provided so as to engage said outer peripheral wall portion at a radially inside surface thereof and to press said outer peripheral wall portion radially outwardly toward said back press roller. The printing drum in such a rotary stencil printer is conventionally constructed in such a structure that a metallic cylinder member having a cylindrical wall is formed with a number of openings through the cylindrical wall thereof for transmitting ink therethrough, said cylindrical wall being constructed to be a plate having a thickness enough to maintain the overall strength and rigidity of the printing drum.
In the rotary stencil printer having the printing drum and the back press roller of the above-mentioned structure ink is supplied to the inside space of the printing drum, and then the ink flows through the openings formed in the cylindrical wall of the printing drum to reach the outside thereof and is supplied to the inside surface of the perforated stencil sheet attached around the cylindrical wall of the printing drum in a cylindrical form, and then the ink flows through the perforation formed in the stencil sheet to reachthe outer side thereof.
In the rotary stencil printer having the above-mentioned conventional structure, for the reasons that the printing drum has the structure of a cylindrical body made of a plate material having a certain substantial thickness, that the ratio of the area allowed for the ink transmitting openings to be bored through the cylindrical body is naturally restricted from the view point of maintaining the strength and the rigidity of the printing drum, and that such a conventional printing drum generally bears two or three windings of a net material made by weaving or knitting filaments and wrapped therearound in order to distribute the respective flows of ink supplied through the mutually apart openings uniformly to intermediate portions between each two adjacent openings, an approaching movement of a substantial amount of ink and accordingly a corresponding substantial time therefor are required before the ink supplied to the inside of the printing drum is uniformly supplied to certain required portions of the stencil sheet mounted over the cylindrical outer surface of the printing drum. Therefore, when a new printing is started with a newly perforated stencil sheet being mounted around the printing drum it is unavoidable to carry out the initial trial printing of at least several printing sheets before a normally high qualaity printing is available.
On the other hand, in the rotary printers, not particularly in the rotary stencil printer, if the printing drum and the back press roller are approached to one another so as to close the nip area when no printing sheet is provided in the nip area, the surface of the back press roller will be stained by the ink on the printing drum. Therefore, it is necessary that the pressing together of the printing drum and the back press roller is only allowed when a printing sheet is provided in the nip area. Therefore, the rotary stencil printer generally incorporates a means for detecting the supply of a printing sheet in the nip area, and in response thereto to drive generally the back press roller toward the printing drum. This is because the back press roller is generally constructed to be smaller and to have less weight than the printing drum, and therefore the back press roller is more suited to be driven quickly in response to the supply of a printing sheet.
Further, in addition to the necessity of the reciprocating movement of the back press roller in synchronization with the supply of the printing sheets, particularly in the rotary stencil printer which has the bar extending along a generatrix of the cylindrical outer surface of the printing drum as an indispensable structural member for holding the leading edge of the stencil sheet to be mounted around the printing drum, because said bar has a structure substantially protruding from the cylindrical outer surface of the printing drum, the back press roller must be moved away from the printing drum to avoid the collision with the bar during the rotation of the printing drum for a distance which is substantially larger than a small clearance necessary for avoiding the back press roller being stained with ink by direct contact with the printing drum. Therefore, in the rotary stencil printer the back press roller is designed to have a much smaller diameter than the printing drum so as the reduce the mass thereof, the nevertheless, because the back press roller must be reciprocated for such a relatively large distance at high acceleration in synchronization with the rotation of the printing drum, the construction for supporting and reciprocating the back press roller needs to have a relatively strong and heavy structure, thereby not only increasing the weight and cost of the printing device but also presenting the problem that relatively high noise is caused in the operation of the printer.